How can I know where the normal ring is convex and where is concave?
This is a Tangent Space normal map.
That is to say, blue (Z) represents the interpolated normal of the underlying geometry, red (X) represents a tilt towards the tangent direction, which is positive X in the UV, (texture,) space, as mapped onto the surface, and green represents a tilt towards the bitangent direction, which is positive Y in the UV space, again, as mapped onto the surface of the geometry.
To quote directly from the Wikipedia page on normal mapping - 'How it Works':

A normal pointing directly towards the viewer (0,0,-1) is mapped to
(128,128,255). Hence the parts of object directly facing the viewer
are light blue. The most common color in a normal map.
A normal pointing to top right corner of the texture (1,1,0) is mapped to
(255,255,128). Hence the top-right corner of an object is usually
light yellow. The brightest part of a color map.
A normal pointing to
right of the texture (1,0,0) is mapped to (255,128,128). Hence the
right edge of an object is usually light red.
A normal pointing to top
of the texture (0,1,0) is mapped to (128,255,128). Hence the top edge
of an object is usually light green.
A normal pointing to left of the
texture (-1,0,0) is mapped to (0,128,128). Hence the left edge of an
object is usually dark cyan.
A normal pointing to bottom of the
texture (0,-1,0) is mapped to (128,0,128). Hence the bottom edge of an
object is usually dark magenta.
A normal pointing to bottom left
corner of the texture (-1,-1,0) is mapped to (0,0,128). Hence the
bottom-left corner of an object is usually dark blue. The darkest part
of a color map.


By Julian Herzog, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=25512888
From this, you can see the top disc is convex, and the bottom disc concave.